It is common for studs, bolts and rods (hereinafter “studs”) to be used as a fastener to provide a secure mechanical connection between structural members, such as, for example, a pair of opposing flanges on a piece of machinery. By placing a preload tension force on the stud, a more secure connection may be created and the overall fastener life may be increased. The tensioning of a stud is typically accomplished by a tensioning system or device that applies an axially-directed force to the stud in a direction away from the structural member. The tensioning system generally includes a mechanism for gripping the stud and a bad cell. An axially-directed force is applied by the load cell to the gripping mechanism. The gripping mechanism transfers the force to the stud, and thereby axially tensions or stretches the stud. The stud is then mechanically retained in its stretched or tensioned position by, for example, a nut that threadably engages external threads formed on the stud and which is tightened down to engage the flange.
Some conventional tensioning systems utilize mechanical load cells, whereas other tensioning systems use hydraulic load cells. Mechanical load cells convert mechanical pressure or force to the axial tensioning force, whereas hydraulic load cells convert hydraulic pressure to the axial tensioning force. Tensioning systems may be configured as either internal gripping, external gripping or integrated tensioning systems where the tensioner is integrated with the fastener.
Consistent with the description provided above, hydraulic tensioning systems typically include a hydraulic cylinder with a puffing feature, such as a puller screw, that attaches to the stud, and a reacting foot that presses against an exposed surface of the flange. An example of an existing hydraulic tensioning system can be seen in FIG. 1, which will be described in more detail below.
In some cases the best connection between a stud and a puller screw is a tapered thread. Tapered thread connections are particularly well suited to uniformly transferring axial load from one member to another. The tapered thread connection is often used when the desired fastener preload is high relative to the stud material strength, such as, 40% to 75% of yield strength. For example, a target preload stress of 60,000 psi may be used for a steel stud with a yield strength of 100,000 psi. A consequence of preloading a stud is that the stress in the stud due to preload results in a strain (i.e., stretch) of the stud material. That is, the preloaded stud is longer than the same stud in the free, unloaded, state. Since the preloaded stud is longer, the threads on the stud have a greater pitch when loaded compared to when the stud is in the free state.
An important factor with tapered thread engagements is that the clearance between the male and female threads can be controlled by adjustment of how far the male thread is screwed into the female. The outer limits of such a fit are a zero engagement and a full engagement. A zero engagement exists when the male thread is not inserted deep enough to contact the female thread. Such a joint cannot transmit load and must be avoided. A full engagement exists when the male thread is inserted into the female to the point of stop, wherein no clearance exists between the flanks of the thread pair. In this instance, there is no allowance for dimensional variation between the male and female threads. While this is the strongest engagement, it is also the least forgiving, as will be explained below.
In practice, operators turn the puller screw in until resistance stops further engagement, as seen in FIG. 2. Once the puller screw is engaged, an axial tensioning load is applied to the puller screw and reacted against by the stud. This load produces a strain in both the puller screw and the stud. The joint is effective at transmitting load, and since no relative movement is required within the joint, there is no issue at this stage of the process. While the system is under hydraulic load, the operator turns a nut down in order to take up the distance that the stud has stretched. The hydraulic load is then released allowing the stud strain to produce preload into the stud. At this point, the stud is under tension and the puller screw in the free state. The female tapered thread of the stud is now stretched and the male tapered thread of the puller screw is relaxed back to its original length. The lack of initial clearance between the thread pair plus the strain differential between the components causes interference between the thread flanks, as seen in FIG. 3. The operator must unthread the puller screw from the stud in order to remove the hydraulic tensioning system and continue work, but typically has difficulty because of the high interference between the thread flanks. The result is that a high removal torque is required to undo the puller screw from the stud. For example, this torque could be 300 to 2,000 ft-lb for a 1.5″ thread joint. This high resistance torque is a hindrance to effective tensioning operation leading to undesired costs of time and money.
In order to alleviate the need to use high removal torque to undo the puller screw from the stud, the thread engagement depth between the puller screw and the stud could be controlled by procedure, such that a net clearance exists between the thread flanks when tensioning starts then allowance for dimensional change can be achieved. However, insertion depth limits are hard for field personnel to control because the environment does not allow for accurate measurement of insertion depth. As such, this method has failed to produce reliable results in practice.
In another prior method, lubrication could be applied to the threads to reduce the frictional resistance when unthreading the puller screw from the stud. However, liquid lubrication tends to squeeze out from the contact areas thus producing no benefit, and dry lubrication can compact within the space between the threads leading to low joint strength. Compacting can be caused by either excessive quantity or repeated application for repeated joint work. As such, the lubrication method has also faded to produce reliable results.
Tight manufacturing tolerances of the male and female threads on the puller screw and stud could also be used, but the cost to reproduce parts to the required tolerances is cost prohibitive to implement.
Accordingly, there exists a need for an apparatus and method for reducing the amount of torque required to decouple the tapered threads of the puller screw and stud. The present invention fills these, as well as other, needs.